Directed Assembly Of Functional Heterostructures

ABSTRACT

The present invention relates to a systematic process for the creation of functionally organized, spatially patterned assemblies polymer-microparticle composites including the AC electric field-mediated assembly of patterned, self supporting organic (polymeric) films and organic (polymeric)-microparticle composite films of tailored composition and morphology; the present invention further relates to the incorporation of said assemblies into other structures. The present invention. also relates to the application of such functional assemblies in materials science and biology. Additional areas of application include sensors, catalysts, membranes, micro-reactors, smart materials. Miniaturized format for generation of multifunctional thin films. Provides a simple set-up to synthesize thin films of tailored composition and morphology:

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on and claims priority of U.S.Provisional Application No. 60/300,025, filed Jun. 21, 2001, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

A longstanding objective within the materials, engineering, biomedicaland analytical sciences has been the design of ever-smaller structuresand devices for use in miniaturize systems capable of performingspecific functions, such as sensors, transducers, signal processors orcomputers. Of particular interest as potential building blocks in thiscontext have been functional materials having predetermined properties.Patterned films composed of suitable polymers and polymer-microparticlecomposite films offer particularly attractive opportunities to realizehierarchically organized structures of functional materials and toprovide confinement and segregation for performing “local” chemicalreactions.

Several methods of preparing patterned polymer films andpolymer-microparticle composite films have been described. In oneexample, polymer molding has been used to prepare polymeric films.Beginning with a master that is fabricated from a silicon (Si) waferusing conventional lithographic techniques, a mold is made using anelastomer such as polydimethylsiloxane (PDMS). The mold is then used toproduce replicas in a UV-curable polymer such as polyurethane. Theapplicability of this technique of polymer molding, long used forreplication of micron-sized structures in devices such as diffractiongratings, compact disks, etc., recently has been extended to nanoscalereplication (Xia, Y. et al., Adv. Mater. 9: 147 (1997), Jackman, R. J.et al., Langmuir. 15:2973 (1999), Kim, E. et al. Nature 376, 581 (1999).

Photolithography has been used to produce patterned, stimuli-sensitivepolymeric films which can be further functionalized with bioactivemolecules and which undergo abrupt changes in volume in response tochanges in pH and temperature (Chen, G. et al., Langmuir. 14:6610(1998); Ito, Y. et al., Langmuir 13: 2756 (1997)). UV-induced patternedpolymerization of various hydrogel structures within microchannels hasbeen described as a means for the autonomous control of local flow(Beebe, D. J. et al., Nature. 404:588 (2000)).

Surface-initiated ring-opening metathesis polymerization followingmicrocontact printing has been used to create patterned polymer layerswhich remain attached to the surface and produce structures ofcontrolled vertical and lateral dimensions (Jeon, N. L. et al., Appl.Phys. Lett. 75:4201 (1999)). Other techniques such as thermal radicalpolymerization (Liang, L., J. Appl. Polym. Sci. 72:1, (1999)) andUV-induced polymerization (Liang, L., J. Membr. Sci. 162:235 (1999))have been used to generate surface confined thin, uniform andstimuli-sensitive polymeric films.

Sarasola, J. M. et al. (J. Electroanal. Chem. 256:433, (1988) and Otero,T. F. et al., J. Electroanal. Chem. 304:153, (1991) describeelectropolymerization of acrylamide gels using Faradaic process.Acrylaminde gels are prepared on electrode surfaces by an anodicoxidative polymerization process using the electroactive nature ofacrylamide monomers.

Polymerization of crosslinked acrylamide has been described to produce amatrix of glass-immobilized polyacrylamide pads which were activatedwith receptor molecules of interest including oligonucleotides orproteins. The use of the resulting porous and highly hydrated matrix forsimultaneous monitoring of ligand-receptor binding reactions has beenreported (Proudnikov, D. et al., Anal. Biochem. 259:34 (1998); Yershov,G., Proc. Natl. Acad. Sci. U.S.A. 93:4913 (1996), LaForge, S. K., Am. J.Med. Genet. 96:604 (2000); Khrapko, K. R. et al. U.S. Pat. No.5,552,270, 1996; Ershov, G. M. et al. U.S. Pat. No. 5,770,721, 1998;Mirzabekov et al. U.S. Pat. No. 6,143,499.).

A process for the assembly of a 3-D array of particles has beendescribed which is based on the synthesis of a core-shell latex particlecontaining a core polymer with a glass transition temperaturesignificantly higher than that of the shell polymer. In accordance withthat process, particles were assembled into a 3-D close packed structureand annealed in such a way that the core particle remained unalteredwhile the shell polymer flowed, resulting in a continuous matrixembedding an organized 3-D array of core particles (Kalinina, O. andKumacheva, E., Macromolecules. 32:4122 (1999); Kumacheva, E. et al.,Adv. Mater. 11:231 (1999), Kumacheva, E. et al., U.S. Pat. No. 5,592,131(1999)).

The encapsulation of a colloidal crystalline array within a thin,environmentally sensitive hydrogel matrix capable of swelling inresponse to changes in pH and temperature has been described. In otherinstances, the hydrogel contained immobilized moieties capable oftriggering the swelling of the gel in the presence of particularanalytes. The swelling of the gel matrix increases the periodicity ofthe colloidal crystal array and produces a shift in Bragg diffractionpeaks in the spectra of the scattered light (Holtz, J. H. et al., Anal.Chem. 70:780 (1998); Haacke, G. et al., U.S. Pat. No. 5,266,238, 1993;Asher, S. A., U.S. Pat. No. 5,281,370, 1994). The process of forming thecolloid crystal relies on passive diffusive transport of particleswithin the prepolymer reactive mixture.

Each of the aforementioned references are incorporated herein byreference in its entirety.

SUMMARY OF THE INVENTION

The present invention provides a systemic synthetic process to translatea sequence of synthetic instructions into a sequence of syntheticoperations that are performed in a homogenous fluid phase, to producepatterned polymeric films, functional polymeric films, multicomponentmicroparticle assemblies and/or polymer-microparticle film composites ofpre-determined composition, layout and morphology. Rather than arrangingindividual molecules by explicit external placement, this approachcombines dynamically controlled “self-assembly” and triggeredpolymerization process to realize heterostructures of preconceivedarchitecture and design.

In one aspect, the present invention provides methods and apparatus forassembling particles at preset times and in predesignated positions on asubstrate surface and to mediate the transformation of thin, patternedgel films. The present invention thus permits a sequence of multiplereaction steps to be executed at preset times in accordance with anexternally set schedules within a homogenous reaction, each stepinvoking an active transport or reaction process.

In another aspect, the present invention provides processes andapparatus for synthesis of patterned polymer films and/orpolymer-microparticle film composites that are mediated by AC electricfield. The present invention also relates to the incorporation of thegels and composites into other structures. The present invention furtherrelates to the application of such gels and composites in materialscience and biology. Illustrative areas of application include:catalysts, smart materials, membranes, sensors and microreactors.

In contrast to some of the methods for producing functionalizedpolymeric films, the present invention does not require complexchemistries of limited applicability nor does it require multipleunrelated processing steps. Furthermore, in the case ofpolymer-microparticle film composite structures, the present inventiondoes not rely on diffusive transport, a slow and environmentallysensitive process, in the assembly of ordered particle arrays.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration showing an experimental configuration forLEAPS.

FIG. 2 (a) contains a photograph showing a patterned gel film and asecond photograph showing a close-up of a section of the film.

FIG. 2 (b) is a photograph showing a free-standing gel film imaged inaqueous phase.

FIG. 3 (a) contains a photograph showing a patterned gel-microparticlecomposite film created via thermal initiation and a close-up of thecentral section of the film.

FIG. 3(b) is a photograph showing a monolithic gel-microparticlecomposite film created via UV-initiation.

FIG. 4 (a) is an illustration showing a flipped gel-particle compositefilm.

FIG. 4 (b) is an illustration showing a flipped gel-particle compositefilm with the particles partially exposed.

FIG. 5 is an illustration showing a cleaved gel-particle composite film.

FIG. 6 is an illustration showing two exemplary processes for produceporous a gel-particle composite film.

FIG. 7. is an illustration showing a process to produce a gel-particlecomposite film by reversible gelation.

FIG. 8. is an illustration showing a process to produceinorganic-organic hybrid films.

FIG. 9. is an illustration showing a process to produce and characterizea magnetic gel-particle composite film.

FIG. 10. is an illustration showing a DNA hybridization assay using aflipped polymer-gel composite film.

FIG. 11. is an illustration showing electrophoretically assisted DNAhybridization.

FIG. 12. is an illustration showing an immunoassay using a flippedpolymer-gel composite film.

FIG. 13. is an illustration showing the analysis of multiple samples ona monolithic gel chip.

FIG. 14. is an illustration showing a process to implement a cell-beadheteroreactor.

FIG. 15. is an illustration showing a heteroparticle arrays.

FIG. 16. is an illustration showing a glucose biosensor.

FIG. 17. is an illustration showing microparticle-encoded vesiclesembedded in a gel film.

FIG. 18. is an illustration showing a gel-embedded cellular array andits use.

FIG. 19 shows the effect of gel chemistry and formation conditions ondiffusion.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods for synthesis of patternedpolymeric films and/or polymer-microparticle film composites that aresimple in implementation and flexible in the choice of polymer chemistryused. Also provided is an apparatus useful in said methods. Patternedpolymer films and polymer-microparticle film composites and their usesare also provided. The invention is based at least in part on thetechnology designated “LEAPS” (which refers to “Light-ControlledElectrokinetic Assembly of Particles near Surfaces).

In certain embodiments, the methods of the present invention combinesthe action of an active self-assembly process acting on long lengthscales such as LEAPS with externally triggered, template-directed gelchemistries to provide: the self-assembly of microparticle arrays indesignated positions on a planar or substantially planar substrate, theexternally directed, sequential execution of multiple assembly stepsrequiring a schedule of initiation and termination; and spatialconfinement to enable concurrent execution of multiple assembly steps indifferent compartments. The resulting heterostructures exhibit anorganization in accordance with a predesigned architecture to meet therequirements associated with the execution of specific functions.Applications of the process to the fabrication of functional materials,sensors and more generally chemical transducers and informationprocessors also are of interest.

Light-Controlled Electrokinetic Assembly of Particles Near Surfaces

LEAPS technology relates to movement of particles and/or fluid suspendedat an electrolyte solution-electrode interface and is described indetail in PCT International Application No. PCT/US97/08159 and U.S.patent No., as well as in U.S. Ser. No. 09/397,793, filed Sep. 17, 1999;U.S. Ser. No. 09/320,274, filed May 28, 1999, and PCT InternationalApplication No. PCT/US00/14957; U.S. Ser. No. 09/813,571, filed Mar. 21,2001; and PCT International Application No. PCT/US01/20179, filed Jun.21, 2001. Each of these patents/patent applications is incorporated byreference in their entirety.

LEAPS involves the use of electrokinetic and polarization-induced forceswhich arise in accordance with the lateral impedance gradients at aninterface between an electrolyte solution and an electrode to controlfluid flow, particle transport and/or particle assembly.

In one embodiment of LEAPS, a plurality of particles are suspended in aninterface between an electrolyte solution and a light sensitiveelectrode (e.g., a planar electrode). An AC electric field is generatedat the interface and the interface is illuminated with a predeterminedlight pattern to assemble particles in the areas of the electrodedesignated by the pattern of illumination (e.g., regions of lowimpedance). Particles move to the low impedance area and form anassembly when the frequence of the applied electric field is less thanthe relaxation frequency of the particles. Accordingly, if therelaxation frequency of the particles are known, one can directly adjustthe frequency of the electric field to form the particle assembly. Ifthe relaxation frequency of the particles are not known, then thefrequency of the applied AC field may be readily adjusted until theassembly occurs. If no particles are present in the interface, and themovement of the fluid is what is desired, that may be accomplished byadjusting the frequency of the applied electric field to be less thanthe relaxation frequency of the electrolyte solution-electrodeinterface.

In another embodiment of LEAPS, a patterned electrode is used instead ofthe light-sensitive electrode. For example, a first electrode ispositioned in a first plane and a second electrode (preferably of planargeometry) is positioned in a second plane different from the first. Theparticles suspended in an electrolyte solution (or an electrolytesolution without the particles) are located between the first and thesecond electrode. The second electrode comprises a patterned electrode.The term “patterned electrode,” as used herein, refers to an electrodehaving a surface and an interior, either or both of which are modifiedto produce spatial modulations in the properties of the second electrodethat affects the local distribution of the electric field at theelectrolyte solution-electrode interface. When the AC electric field isapplied at the interface, the particles assemble in designated areas ofthe second electrode that are defined by the spatial modulations in theproperties of that electrode (e.g., low impedance regions). In theabsence of the particles, fluid movement may also be controlled by theapplication of AC field.

The electrode may patterned by a number of ways to affect theinterfacial impedance. Preferably, the electrode is patterned byspatially modulated oxide growth, surface chemical patterning or surfaceprofiling. If the patterned electrode is a light-sensitive electrode,the patterning in combination with the illumination pattern on saidelectrode may be used to control the movement of the particles and/orfluid at the solid-liquid interface.

In preferred embodiments, the patterned or light-sensitive electrodecomprises a silicon electrode (e.g., silicon chip), which may also becoated with a dielectric layer. One such example is a Si/SiOx electrode.In one particularly preferred LEAPS configuration, an additionalelectrode is provided such that the light-sensitive (or the patternedelectrode) and the additional electrode are substantially planar andparallel to one another and separated by a gap (e.g., in a sandwichconfiguration), with the electrolyte solution (with or without theparticles) being located in the gap. The additional electrode preferablycomprises an optically transparent electrode (e.g., ITO coated glass),which allows optical inspection of the movement of the particles and/orfluid at the interface. When such type of LEAPS cell is used, an ACelectric field may be applied at the solid-liquid interface by applyingan AC voltage between the light-sensitive (or patterned) electrode andthe additional electrode.

When a plurality of particles are suspended in the electrolyte solutionand subject to LEAPS, it is preferred that the particles form a planarassembly on the designated areas of the electrode, more preferably in anarray configuration. However, the particles may also be assembled in alinear configuration or any other configuration that is dictated by theillumination pattern and/or patterning.

The term “particles” as used herein include colloidal particles,eukaryotic and prokaryotic cells, micells, vesicles (e.g., liposomes)and emulsion droplets. In preferred embodiment, the size of theparticles range from about 0.2 to about 20 μm in diameter.

Formation of Patterned Polymeric Film

The present invention provides methods for synthesizing patternedpolymeric film using the LEAPs technology described in the precedingsection. In certain embodiments, a polymerization mixture comprising amonomer and an initiator in an electrolyte solution is provided.Preferably, the polymerization mixture also contains a cross-linker,with the monomer, initiator and the crosslinker dissolved in theelectrolyte solution. This mixture is placed between the light-sensitive(or patterned) electrode and the additional electrode. An AC electricfield is applied in the interface between the electrolyte solution andthe light-sensitive (or patterned) electrode. Lateral impedancegradients at the interface, set up by the patterning or thepredetermined pattern of illumination, give rise to local recirculatingelectro-osmotic fluid motion, which effectively transports fluid (andparticles if they are present) from regions of high impedance to regionsof low impedance. Depending on the initiators used, the application ofthe AC electric field, in addition to the illumination of the electrodewith a predetermined light pattern (when light-sensitive electrode isused) or the patterning of the electrode, may be sufficient to induceformation of a patterned polymeric film on the low impedance regions ofthe light-sensitive or patterned electrode.

In preferred embodiments, the polymerization is triggered at a desiredtime by using initiators that are heat or photoactivated. If such acase, the polymerization mixture is heated or irradiated with UV-lightto initiate polymerization. Heat-generated or UV-generated free radicalsdiffuse and react with monomers to produce initially oligomers andfinally a crosslinked polymer film.

As the gel film grows, a moving reaction extends into the solution withtime. In case of the heat-induced polymerization, polymerization startsfrom the light-sensitive or patterned electrode (e.g., silicon chip).Due to the presence of LEAPs-mediated, strong convective transport nearthe light-sensitive (or patterned) electrode surface, the polymerizationprocess is triggered preferentially in the low impedance areas on thatelectrode, thereby giving rise to a spatially patterned polymeric filmon said electrode. In case of UV-induced polymerization, however,polymerization starts at the additional electrode (usually the topelectrode), and produces an unpatterned monolithic gel.

The present invention, in contrast to several known methods, do notrequire complex implementation, such as use of a mask, in preparation ofpatterned gel films. In addition, the methods of the present inventionallow increased flexibility in choice of monomers, crosslinkers andinitiators used. It should, however, be noted that high viscosity of thepolymerization mixture and high ionic concentration may impede with theproper functioning of LEAPS by interfering with the interfacial fluidflow. Accordingly, it is recommended that the ionic concentration of thepolymerization mixture be about 1.0 mM or lower, preferably betweenabout 0.1 mM to 1.0 mM. This may be accomplished by selecting initiatorsto maintain low ionic concentration of the mixture. Initiators, as aremonomers and crosslinkers, are well known in the art and may readily beobtained from commercial sources.

As for the monomers and crosslinkers, it is recommended that lowviscosity monomers and crosslinkers be used, such that the viscosity ofthe polymerization mixture is about 100 cp or less. When the patternedfilm to be produced is a hydrogel, water-soluble monomers are preferred.In addition, when said film is optically transparent. The desiredmonomer concentration may be adjusted according to the type of gel to beproduced (e.g., self-supporting or cleaved gel). In one embodiment, amixture of acylamide and bisacrylamide of varying monomerconcentrations, from about 20% to about 5%.(acylamide:bisacrylamide=37.5:1, molar ratio) may be used to produce ahydrogel. In preferred embodiments, the polymeric film obtainedcomprises a cross-linked alkylacrylamide or hydroxyalkymethacrylatehydrogel.

The AC voltage depends on the polymerization mixture and is readilyadjusted until the desired polymeric film (or polymer-microparticle filmcomposite) is formed. Preferably, the voltage applied is in the range ofabout 0.5 to about 15 V (peak to bead) and the frequency is preferablymore than about 10 hz and less than about 500 kHz, more preferably about1 kHz to 10 kHz.

In one embodiment of the invention, LEAPS is carried out in a fluidicmicrocell formed by sandwiching a double-sided Kapton spacer of about100 um thickness (between a 1 cm×1 cm silicon chip (n-typed, cappedeither by a uniform or a lithographically patterned thin SiO2 layer) anda glass cover slip coated with indium tin oxide (ITO) to a typical sheetresistance of 1400 Ohn square.

In preferred embodiments of the present invention, an electrolytesolution (more preferably, an aqueous solution) is used in thepolymerization mixture, e.g., to dissolve monomers, crosslinkers andinitiators. In certain embodiments, other polarizable liquid medium maybe used, including non-aqueous solution. The relaxation frequency of theparticles assembled in a non-aqueous solution (e.g., DMSO andacetonitrile) is shifted to lower values when compared with that of anaqueous solution.

The hydrogels of the present invention may be functionalized by varietyof methods known in the art. For example, during the polymerization stepitself small amounts of functional monomers may be introduced along withthe polymerization mixture (e.g., acrylamide mixture). Acrylic acid,2-hydroxyethylmethacrylate (HEMA), diethylaminoethylmethacrylatehydrochloride etc. could be incorporated into the hydrogel so that themicropatterned gel may be chemically addressed via the carboxy, hydroxyand amino functional groups. Biomolecules of interest may subsequentlybe immobilized in the gel using suitable chemistry and linker molecules.

Small probe molecules or functional co-monomers may also be introducedinto the hydrogel using the same approach to yield novel sensor andstimuli responsive hydrogel structures, that can respond to a variety ofinputs such a pH, temperature, electric field, light etc. Microscalestructures made from such stimuli-responsive materials may act as anactuator, for example for controlling fluid flow (valve). Suchstructures will be self regulating and would not require an externalpower source.

Polymer-Microparticle Film Composites

By providing a plurality of particles suspended in the polymerizationmixture, the methods for patterned polymeric film synthesis, asdescribed in the preceding section, may be used to obtain an assembly ofthe particles embedded in a polymeric film (also referred to as“polymer-microparticle composite” or “heterostructure”). The compositeformation is comprised of two stages. First, particle assemblies (e.g.,planar particle assemblies, more preferably particle arrays) are formedfrom the particle suspension also containing all ingredients requiredfor subsequent in-situ gel formation, as described previously. Second, apolymeric film is formed to produce the polymer-microparticle filmcomposite. In one preferred embodiment, gels are formed byheat-initiated in-situ polymerization to form a composite in which thegels are spatially patterned. In another preferred embodiment, the gelsare formed by UV-initiated in-situ polymerization to obtain a compositein which the gels are monolithic (not patterned).

In one embodiment, AC voltage of 1 to 20 V p-p in a frequency range offrom about 100's of hertz to several kilohertz are applied between theelectrodes across the fluid gap. Fluid and particle transport andassembly may be monitored by video microscopy permitting frame captureand digitization of frames for further analysis.

The thermal free radical polymerization may be initiated by heating thepolymerization mixture (e.g., by heating the LEAPs cell), for example,to about 40 to 45 C, for about 1 to 10 minutes, using an IR lamp, whilemaintaining the AC electric field at the electrolyte solution-electrodeinterface, to form a patterned film or polymer-microparticle filmcomposite.

The polymerization may also be triggered by irradiating thepolymerization mixture with UV-light. For example, in the presence ofthe applied AC electric field, polymerization may be triggered by usinga mercury lamp source. A wide range of wavelengths spanning from about250 to 340 nm may be used, for about 15 seconds to about 10 minutes. Inone preferred embodiment, the concentration of monomers in thepolymerization mixture may be about 10% by weight, and2-hydroxy-4′-hydroxyethoxy-2-methylpropiophenone) may be used as theinitiator to give a 1.5% by weight solution.

In certain embodiments, particles comprise beads (also referred to as“microparticles” or “microspheres”) that are composed of silica,modified polystyrene or other polymers. Preferably, these particles areanionic or cationic particles ranging from about 0.5 μm to about 15 μmin diameter. In certain preferred embodiments, these particles arefunctionalized by attaching a variety of chemical functional groups totheir surfaces. The process of forming composite gel-particle films mayreadily be extended to particles that display biomolecules attached ontheir surfaces, such as receptors or ligands. In certain embodiments,oligopeptides, proteins, oligonucleotides or nucleic acid fragments mayalso be attached to the particle surfaces. The particles may also beencoded by use of a chemically or physically distinguishablecharacteristic that are uniquely identifies the biomolecules attached tothose particles, an example of which includes color encoding theparticles using fluorophore or chromophore dyes. Such a process allowschemical immobilization of functionalized microparticle assemblies orarrays for a variety of biochemical assays, including binding andfunctional assays. Examples 6 to 9 describe a number of these assays.

In certain embodiments, the particles used in preparingpolymer-microparticle film composites may be magnetic particles. Incertain other embodiments, the particles used are eukaryotic orprokaryotic cells, or liposomes. The polymer-microparticle filmcomposites produced using these particles may also be used in variousbiochemically assays, including the assays described in Examples 12 to16.

The particles useful in preparation of the polymer-particle filmcomposite may also comprise inorganic particles, including metalparticles, semiconductor particles and glass particles. The inorganicparticles may also be coated with a polymeric shell.

Self-Supporting, Flipped and Cleaved Gels and Gel-Microparticle Films

Accordingly, the present invention provides novel patterned films and/orpolymer-microparticle film composites, including a planar assembly orarray of particles embedded in a gel (2-dimensional assembly). Inpreferred embodiment, these gels are prepared according to the methodsdescribed above.

As discussed previously, the patterned polymeric films and thepolymer-microparticle film composites of various types may be produced,for example, by varying the monomer concentration.

In one embodiment of the present invention, a self-supporting film(preferably a hydrogel) is prepared. In one example, the concentrationof monomers in the polymerization is greater than about 10% by weight.Preferably, acrylamide monomers are used. Following the polymerization,the LEAPS microcell may be dismantled with the gel matrix attached tothe light-sensitive (or patterned) electrode. The hydrogel produced isself supporting and free standing patterned gel films may be obtainedsimply by peeling it off from the electrode. The film is stable inaqueous solution and stays intact for months. An example of such a freestanding gel is shown in FIG. 2(b).

In addition to the substrate-supported and self-supporting gel filmsdescribed above, a “Lift-Off” processes may be used to obtain polymericfilms and/or composites that are detached from the light-sensitive (orpatterned) bottom electrode. In one example, a vinyl siloxane coated ITOcoverslip is used as an electrode for the LEAPS assembly cell. The vinylsiloxane coating allows covalent tethering of the gel film on the ITOelectrode. Beads, suspended in a solution containing all ingredientsrequired for subsequent in-situ gel formation, are assembled indesignated regions of the light-sensitive (or patterned) electrode usingan AC-electric field at a given voltage and frequency.

Keeping the field switched on, the LEAPs cell may, for instance, beirradiated with UV-light from a 150 W Hg source for ˜3 minutes.Afterwards, the UV illumination and field are switched off and the LEAPScell is opened by separating the bottom silicon electrode from the topITO electrode: the covalent attachment of the gel to the top electrodeensures that the gel remains adhered to the top electrode and readilyseparates from the bottom electrode. By “flipping” thesubstrate-attached gel film, beads displaying receptors capable ofbinding the molecules of interest are located at the outer, exposedsurface of this “flipped” gel (“FlipGel”). Thus, the diffusion length ofthe molecules to migrate from the solution above the gel to the beadsurface is small compared to that in the case of regular gels (see FIG.4 (a)). An assay is then conducted on the gel-embedded bead array byexposing the gel to the solution containing analyte molecules ofinterest.

In certain other embodiments, the position of the bead array relative tothe outer bounding surface of the embedding gel film may be controlledby assembling the microparticle array on a topographically patternedelectrode surface exposing designated recesses of defined depthcontaining a non-aqueous phase that is non-miscible with an overlaidaqueous phase containing the microparticles as well as the chemicalconstituents required for gel film formation in accordance with theprevious protocols (see FIG. 4 (b)). Upon application of the requisiteAC electric field, microparticles assemble within the designatedrecesses in such a way as to permit particles to remain partiallysubmerged within the organic phase deposited into the recesses prior toassembly. Following assembly, gel formation is initiated in the mannerdescribed; however, the immiscibility of the two layered phases ensuresthat polymerization is confined to the aqueous phase, thereby leavingembedded microparticles partially exposed.

In certain other embodiments, a cleaved gel is prepared, following thesame principle as FlipGels. The basic differences are that a) themonomer concentrations used in the polymerization reaction are smaller(for example, 5% by weight) and b) the time of irradiation is shorter.Under these conditions, the degree of polymerization is not uniformthroughout the thickness of the cell. Typically, the degree ofpolymerization and crosslinking is highest near the top electrode (e.g.,ITO electrode) and progressively grows weaker as one approaches thebottom electrode (e.g., silicon chip). After gelation, on disassemblingthe LEAPS cell and pulling the two electrodes apart, such a geltypically fractures at a plane very close to the substrate surface (seeFIG. 5). Thus, a layer of gel remains attached to the ITO-coatedcoverslide while the silicon retains the rest of the gel containing theassembled bead arrays. The silicon chip can now be used for a variety ofassays with the assay solution location directly on top of the gel. Inthis case, the diffusion length of the molecules is reduced from that ofa regular gel because the cleavage usually occurs just over the planecontaining the bead array, leaving beads more accessible to moleculespresent in the solution above the gel.

Gels of the present invention may be porous. Polyacrylamide gels, forexample, have typical pore sizes ranging from a few nm to 15-20 nm inhighly diluted formulations. To facilitate the penetration of large DNAfragments and other molecules into gels, macroporous polyacrylamides maybe prepared by polymerizing in the presence of preformed polymers suchas poly(ethylene glycol)(PEG), polyvinyl pyrrolidone (PVP),hydroxymethyl cellulose (HMC) etc. (Righetti, P. G. and Gelfi, C. 1996.J. Chromatogr. B. 699: 63-75.). Highly hydrophilic monomers, such astrisacryl may also be used to produce highly porous gels (Gelfi, C., etal. 1992. J. Chromatogr. 608: 333-341). FIG. 6 illustrates the protocolto form a porous gel using preformed polymers.

Reversible Immobilization of Microparticles within Gel Films

So far, the process of forming polymeric films and polymer-filmcomposites involved synthesis of chemically crosslinked polymers. Theprocess of forming composite gel-particle films can, however, easily beextended to include physically gelling systems such as block copolymergels, agarose gels, gelatin gels etc. Such gels consist of polymericnetworks held together by physical rather than chemical crosslinking.The reversible gelation of such systems may, for example, be triggeredthermally with the system existing as a sol at a high temperature andtransforming into a gel on cooling and vice versa. The reversibility andthe capability to form and immobilize bead arrays on cue allows to carryout a on-chip bioassay dynamically. An example of such a scheme is shownin FIG. 7.

EXAMPLES

The present invention will be better understood from the ExperimentalDetails and Examples which follow. However, one skilled in the art willreadily appreciate that the specific methods and results discussed aremerely illustrative of the invention described in the claims whichfollow thereafter.

Example 1 AC Electric Field-Mediated Formation of Patterned Gel Films

LEAPS is carried out in a fluidic microcell formed by sandwiching adouble-sided Kapton spacer of ˜100 μm thickness (between a 1 cm×1 cmsilicon chip (n-type, capped either by a uniform or a lithographicallypatterned thin SiO₂ layer), also serving as the bottom electrode, and aglass cover slip coated with indium tin oxide (ITO) to a typical sheetresistance of 1400 Ohm Square serving as the top electrode. FIG. 1illustrates the various components of a LEAPS microcell.

The mixture of monomers and the initiator is introduced within the LEAPScell and the electric field is applied. The thermal free radicalpolymerization is then initiated by heating the cell ˜40-45° C. using anIR lamp (the polymerization can also be triggered by a step change inthe bias voltage from a large positive value to a small positive value).Typical parameters of the AC electric field used for this particularexample are V_(p-p)˜5-8V and ω˜1 kHz. This AC electric field-mediatedprotocol leads to the formation of a thin layer of hydrogel inpredesignated areas (low impedance regions) on a Si/SiO_(x) substrate.

Hydrogels are formed using azodiisobutyramidine dihydrochloride as athermal initiator at a low concentration ensuring that the overall ionicstrength of the polymerization mixture falls in the range of ˜0.1 mM to1.0 mM. The hydrogels are composed of a mixture of acrylamide andbisacrylamide of varying monomer concentrations from 20% to 5%(acrylamide:bisacrylamide=37.5:1, molar ratio).

FIG. 2 illustrates a hydrogel formed on an interfacially patternedsubstrate under the action of electric field. The gel is formedexclusively in the low impedance regions (thin oxide) of the substrate.The wrinkled pattern seen on the hydrogel surface is caused by amechanical instability set up in the gel during polymerization (Tanaka,T. 1987. Nature. 325:796; Warren, J. A. 1995. Spatio-Temporal Patterns,Ed. Cladis, P. E. and Palffy-Muhoroy, Addison-Wesley. 91-105).

Example 2 Preparation of Gel-Microparticle Hybrid Films

Two stage process is used to synthesize polymer-microparticle filmcomposites. First, ordered particle arrays are formed from amicroparticle suspension also containing all ingredients required forsubsequent in-situ gel formation in accordance with Example 1. LEAPS(see Example 1) is invoked to form arrays from particles suspended in alow viscosity monomer(s) dispersion mixed with an initiator inaccordance with Example 1. Second, gels are formed, either viaheat-initiated in-situ polymerization (Example 1) to form spatiallypatterned hybrid gels (see FIG. 3(a)) or via UV-initiated in-situpolymerization to form monolithic hybrid gels (see FIG. 3(b)), asdescribed below.

To assemble particle arrays, AC voltages of 1-20 V_(p-p) in a frequencyrange from 100's of hertz to several kilohertz are applied between theelectrodes across the fluid gap. Fluid and particle transport andassembly are then monitored by video microscopy permitting frame captureand digitization of frames for further analysis.

Prior to assembly, particles stored in buffer are centrifuged and washedwith deionized and ultrafiltered (conductivity <50 S cm⁻¹) distilledwater three times. At the last wash, the monomer/crosslinker andinitiator solution is added in amount so as to maintain the originalconcentration of particles. The initiator and/or the salt concentrationis maintained at <=1 mM. The resulting particle suspension is applied tothe LEAPS cell so as to fill the gap between the two electrodes. Anionicand cationic particles ranging from 0.5 μm to 15 μm in diameter,composed of silica, modified polystyrene or other polymers andfunctionalized with a variety of chemical surface groups, as well asfunctionalized core-shell particles obtained from a variety ofmanufacturers are used.

Following array assembly, and in the presence of the applied AC voltage,polymerization of the fluid phase is triggered, for example by using amercury lamp source, to effectively entrap the particle array within thegel. A wide range of wavelengths spanning from about 250 nm to about 340nm is suitable for the polymerization. FIG. 3 shows an example of aparticle array immobilized in a polyacrylamide matrix. The concentrationof the monomers was 10% and the initiator used was a UV initiatorIrgacure 2959® (2-Hydroxy-4′-hydroxyethoxy-2-methylpropiophenone, CibaGeigy, Tarrytown, N.Y.). The initiator was added to the monomer to givea 1.5% by weight solution.

Example 3 Patterned Inorganic Materials

The ability to grow complex materials with small feature sizes is ofmuch interest for the fabrication of structured and multifunctionalfilms, biologically relevant heterostructures and photonic materials foroptical and optoelectronic applications. Thus, processes to formpatterns rapidly and directly to give geometrically as well asfunctionally organized structures without using complicated etchingprocess or complicated chemical schemes can be extremely useful. Inaccordance with the present invention, the LEAPS-directed formation ofpatterned gel and gel-particle composite films provides for thefabrication of a variety of inorganic-organic, organic-organic, or fullyinorganic composite structures.

Organic-Organic Composite—

After formation of the patterned gel film on the low impedance areas ofthe substrate, the high impedance or the silicon oxide capped regions ofthe substrate can be decorated with a second polymer preferably througha process other than bulk radical polymerization (employed to synthesizethe gel); for example covalent modification with silane polymers oroligomers, polyelectrolyte adsorption, hydrophobic interaction, hydrogenbonding etc. Following such a process the earlier gel layer can belifted off, enabling the formation of complementary patterned polymer orgel film.

Organic-Inorganic Composite—

FIG. 8 outlines the scheme of the basic procedure for making metal (Au,Ag, Cu etc.), metal oxide (Fe₂O₃, Co₃O₄, NiO) or semiconductor (CdS,PbS, ZnS) nanoparticles in the patterned gel matrix. The processinvolves exposing the patterned gel on a substrate to a solution of ametal salt, followed by DI water rinse and exposure to reducing agent(in case of the metal) or second salt solution in other cases. Thenucleation and growth of the nanoparticles take place within thehydrophilic domains defined by the gel film.

Inorganic Composite—

Fully inorganic structures can easily be generated from the structuresgenerated above by calcining at high temperatures so as to burn off theorganic component.

Example 4 Interconnections

The realization of interconnections in the form of electrical, opticalor chemical conduits in small devices represents a critical aspect ofthe realization of integrated electronic, optoelectronic or biochemicalprocessors. The methods of the present invention permit the assembly oflinear microparticle assemblies in accordance with LEAPS, either underillumination or on patterned EIS interfaces, and their subsequentimmobilization, for example by embedding within a gel matrix asdescribed herein.

Electrical Conduit—

Following completion of the assembly of metal core/polymer shellparticles into linear configurations, rapid heating of the siliconsubstrate, for example by exposure to pulsed laser light, will melt awaythe polymer components and fuse adjacent metal cores. Of interest inthis application will be particles containing solid metal (Cu, Ni) coresor particles containing metal nanoclusters dispersed into a polymermatrix which may be prepared by methods known to the art.

Optical Conduit—

Within a linear assembly of glass particles, illuminated with focusedlight, particles will guide scattered or emitted light to theirrespective nearest neighbors. Thus, individual beads may be illuminatedby focused laser light and can serve as secondary sources to illuminateadjacent particles within the linear assembly.

Chemical Conduit—

Following completion of the assembly of polymer particles into linear,circular or other desired configurations, particles are permanentlyimmobilized on the substrate, for example by non-specific adsorption,this structure serves as a positive mold around which a gel matrix isgrown which is the lifted to reveal complementary negative surfacerelief; such structures can be closed by fusion with a substrate oranother gel and can serve as linear conduits for the transport ofbiomolecules or other materials.

Example 5 Self-Supporting Magnetic Gel Films

Free standing gel microparticle hybrid films similar to those describedin the detailed description section are prepared using functionalizedand superparamagnetic microparticles or a mixture of superparamagneticparticles with (non-magnetic) color-encoded and functionalizedmicroparticles. Incorporation of magnetically responsive particlespermits the separation of the gel film from a solution containingbiological sample by application of a magnetic field.

This is of particular benefit in carrying out multi-step biologicalassay protocols.

In a protocol enabled by the self supporting magnetic gel films of theinstant invention (FIG. 9A), an in-tube binding assay probing analytemolecules present in solution by permitting capture to bead-displayedreceptors is performed under conditions permitting the magneticgel-microparticle film to remain in suspension (FIG. 9B). Followingcompletion of the assay, magnetic separation (FIG. 9C), achieved byapplication of a magnetic field, permits the temporary immobilization ofthe gel film on a transparent surface of the reaction chamber. Followingfluid and/or buffer exchange, all excess fluid is removed in the laststep, leaving the hydrated gel film exfoliated on the transparentsurface even in the absence of the magnetic field (FIG. 9D). Imagesrecording the results of the binding assay may now be obtained using amicroscope. In a preferred embodiment, a coverslip is positioned abovethe film to prevent evaporation which may lead to buckling of the film.

A combination of magnetic oligo (dt) and antibody functionalized gelmatrix may also be used to carry out simultaneous capture of targetcells to gel via cell-surface antigens, followed by lysing of the celland capture of genomic DNA to magnetic and oligonucleotidefunctionalized microparticles within the gel.

Example 6 Hybridization Assay in Gel-Microparticle Hybrid Films

DNA hybridization assays is conducted using Oligo probe (short singlestranded DNA fragments) functionalized particles embedded in gels. Theprobe coated particles are made as follows. Neutravidin coated beads arewashed thoroughly in salinated PBS of pH 7.4. The biotinylated probesare then added to the bead suspension and mixture incubated at roomtemperature for 90 min. The probe-coated beads are then stored in PBSsolution containing 0.01% Triton.

The targets for DNA hybridization reactions can be eithersingle-stranded or double-stranded molecules. Single-stranded DNA of agiven length and sequence were synthesized chemically (Integrated DNATechnologies, Coralville, Iowa). Double stranded DNA is a PCR-amplifiedproduct directly obtained from genomic DNA of patient samples. The PCRproduct is produced using fluorescence-labeled primers. Afterpreparation, the primers are removed by a PCR purification kit (Qiagen)and the resultant solution can be used in the assay. Single stranded DNAcan also be prepared from double stranded sample by digesting theantisense strand. For this purpose the antisense primers used in PCRamplification have to designed with a phosphate group at the 5′ end. Astrandase enzyme is then used to digest the antisense primer. In eithercase, the DNA at the end of the process in suspended in Tris-EDTA bufferand the concentration is determined using UV optical densitymeasurements.

Before hybridization, the double stranded DNA has to be denatured toyield single strands. For this, the DNA is diluted with Tris EDTA bufferand heated in a sand bath at 95 C for 1 min. It is stored in ice beforeuse. It is then mixed with an equal volume of tetramethylammoniumchloride to yield a desired concentration of DNA for the reaction.

Two types of beads, internally stained with different fluorescent dyesand each bearing a different probe, are used for the reaction. One ofthe probes used is a prefect match with the target strand while theother sequence represents a deletion of 3 bases.

The beads are washed three times with distilled water and finallysuspended in 5% monomer solution and initiator concentration asdescribed earlier. The beads are assembled into arrays in a LEAPS cellusing 4 V peak to peak AC voltage and frequency 500 Hz. After assembly,the cell is irradiated with UV light for ˜3 min. This will yield a FlipGel which is then used for hybridization. The Flip Gel is attachedgel-side up to a polished silicon wafer using single-sided tape. 1 ul oftarget containing 100 ng/ul dna was diluted using 24 ul of TE and 25 ulof 2×TMAC. From the resultant solution 10 ul was added to the gel forreaction. The wafer was enclosed in an air-tight wafer holdingcontainer, sealed and set on a shaker at 50 rpm in an oven at 55 C. Thereaction was conducted for 30 min. At the end of the procedure, the gelwas washed twice in 1×TMAC equilibrated at 55 C.

The gels are prepared for imaging by applying a coverslip on them.Images are taken in the bright field and the Cy5 channels (probeslabeled with Cy5). To distinguish the two different types of particlesin the arrays, images are also taken at two other color channelsappropriate for the internal encoding dyes. The set of four images arethen analyzed to yield the assay results (see FIG. 10)

Example 7 DNA Electrophoresis and Hybridization in Gel-MicroparticleHybrid Films

One method of performing rapid nucleic acid hybridization assays in thegel-microparticle hybrid films involves the use of D.C. electric fieldsto induce electrophoresis of target nucleic acid strands. This isespecially relevant in case of large target fragments whose diffusioninside the gels are expected to be low. Typically the samples foranalysis are denatured and electrophoresed through the gel-microparticlehybrid films, as the complementary single-stranded nucleic acid targetscontact the capture probe (oligo) functionalized beads, they hybridizeand are quantitatively immobilized on the microparticle surface. Thenon-complementary strands does not hybridize with the capture probe andmigrate through the gel unimpeded. The hybridization is detected usingluminescent labels associated with the sample nucleic acid. FIG. 11 twodifferent possible geometries for carrying out electrophoreticallyassisted hybridization in gel-microparticle hybrid films.

Example 8 Immunoassay in Gel-Microparticle Hybrid Films

Protein assays are readily performed on supported gel s, self-supportinggels, Flip Gels and Cleaved Gels. An example of immunoassays performedis the binding reaction between Mouse IgG and Goat Anti-Mouse IgG. Forthis reaction, the beads used in the reaction are surface-coated withthe Mouse IgG. For this purpose, nutravidin-coated particles of size 3.2μm are incubated overnight with the Mouse antibody (SigmaChem) in aphosphate buffer solution of pH 7.2. After the coating process, theparticles are washed thoroughly with PBS containing bovine serumalbumin.

The target molecules of goat anti-mouse IgG are labeled with amonofunctional fluorescent dye Cy5.5 (Amersham). TheNHS-ester-containing dye attaches to the amine groups of the IgG byfollowing a manufacturer supplied protocol. The dye and the IgGmolecules are incubated for 1 hr at pH 9.3. The free dye is thenseparated from the labeled molecules using a gel filtration column andphosphate-buffered saline as the separation buffer. The concentration ofIgG in the sample and the number of dye molecules per molecule of IgG iscalculated.

Two types of particles are used for the reaction, one for the assay andthe other as a negative control. They are distinguished by the use ofinternal encoding dyes which have excitation and emission at differentwavelength from those of Cy5.5. One of the types of particles is coatedwith Mouse IgG as described above and the other has merely a coating ofnutravidin. A mixture of these two types is spun down and washed withD.I. water containing 0.01% Triton three times. After the last spin, theparticles are suspended in the monomer mixture containing 10% monomersolution and the UV-initiator in amounts described earlier. Theparticles are assembled in a LEAPS cell and irradiated to form amonolithic gel. Depending of the concentration and the time ofirradiation, a regular, Flip Gel or Cleaved Gel is formed.

The gel is placed with the support (coverslide in case of Flip Gel,silicon chip in case of regular and Cleaved Gels) gel side up. A givenvolume (10 μl) of a known concentration of the Goat anti-Mouse IgGplaced on the gel. The gel with the solution is then enclosed in anairtight container and put on a shaker operating at 50 rpm in an oven at37 C for one hour. After binding has occurred, the gel os loaded with 20μl of alkaline SDS (Tris base containing 10% SDS) for 30 min to reducenonspecific binding. The gel is then washed with alkaline SDS twice andprepared for imaging. A coverslip is placed on the wet gel and imagesare taken in the bright field, and the Cy5.5 channel. To distinguish thetwo different types of particles in the arrays, images are also taken attwo other color channels appropriate for the internal encoding dyes. Theimages are then analyzed to establish the mean binding intensity and theintensity distribution of each type of bead in the mixture (see FIG.12).

Example 9 Bioanalytical Assay with Integrated Filtering and SpecificCapture

The gel-microparticle hybrid film is ideal for selectively capturingspecific nucleic acids or proteins from a crude mixture like whole bloodor cell lysate. Typically a crude sample containing whole blood iscontacted with the gel containing microparticles functionalized withcapture probe molecules of interest. The red and white cells areautomatically screened by the gel on the basis of their size. Thecomplementary components from plasma bind to the capture probe coatedbeads. Non-complementary components can then be easily washed off.

Example 10 Recording of Assay Images from Hybrid Films

In accordance with the methods of the present invention, a Nikon EclipseE-600FN epifluorescence microscope equipped with 150 W xenon-arc lampand a Nikon 20x 0.75 NA air objective fitted with an optimized set offilter cubes for the selection of fluorophores was used for allmeasurements. Images were recorded with a cooled 16 bit CCD camera(Apogee Instruments Inc.). The exposure/integration times for thevarious preparations varied between 25 ms to 500 ms. User interfacedprograms for analysis of images and assay results were developed usingMATLAB which was run on PC. Image collection and analysis may then beperformed.

Example 11 Multiple Samples Per Chip

FIG. 13 illustrates a method of carrying out multiplexed assays formultiple samples using the same monolithic gel film containing multiplebead arrays. A gel film containing bead arrays is synthesized (asdescribed in Example 3) on an interfacially patterned silicon chip intowhich through holes have been made at four corners (choice of thisgeometry is arbitrary and is chosen here for illustrative purposes only,in principle a wide variety of designs and number of holes can bechosen). The samples are added by pipetting through the back of thechip, and the sample is allowed to spread diffusively and react with thesurrounding particles as shown in FIG. 13. Depending on the length ofthe incubation time the area of the reacted patch will vary (Area ˜tD,where t reaction time and D diffusion coefficient of the target in gel).

Example 12 Cell-Based Heteroreactor

A cell-bead heteroreactor is constructed on a silicon substratecontaining etched through-holes serving as fluidic interconnects. First,a gel-microparticle composite film is formed in accordance with Example3 in the fluidic compartment defined by (the front side of) the siliconelectrode and the ITO-coated glass electrode. Next, suspensions of cellsare introduced into the tapered etched through-holes on the backside ofthe silicon electrode. Molecules secreted from cells within thesemicrowell structures are now allowed to diffuse into the gel where theyare detected by capture to functionalized beads within the previouslyassembled array. Alternatively, cells within the microwells may belysed, and released genomic DNA may be enzymatically fragmented to allowsufficiently small fragments to diffuse into the gel where they arecaptured by hybridization to functionalized beads within the previouslyformed array while large constituents of the lysate are kept out. Thissecond structure can remain open, and may be fashioned to exhibit thedimensions and form factors, for example of a 1536-well microplates;alternatively, a second fluidic compartment may be formed by (the backside of) the silicon electrode and a third delimiting planar substrateto permit microfluidic transport of cell suspensions.

Example 13 Co-Assembly of Heteroparticle Arrays

LEAPS enables the co-assembly of a binary mixture of smaller beads alongwith larger assay beads in designated areas of the substrate (FIG. 15).Once arranged in an array format the smaller beads then undergotwo-dimensional crosslinking since they contain either complementarycharge or reactive groups. The two-dimensional crosslinked aggregatecreated in the process acts as an inert mold for the larger assay beadswhich are thus immobilized. The advantages of the protocol include theease of implementation, control of spatial localization and goodimmobilization efficiency.

Example 14 Fabrication of an Enzyme Sensor by Directed Self-Assembly

In accordance with the methods of the present invention, the combinationof LEAPS-mediated active assembly of an array of functionalizedmicroparticles and the chemical synthesis of a polymeric gel filmpermits the in-situ synthesis of a variety of sensors.

Thus, given a fluidic microreactor composed of patterned silicon/siliconoxide chip and ITO-coated glass electrodes arranged in sandwich geometryas illustrated in FIG. 1, a glucose sensor based on a gel-microparticlecomposite film is constructed by the following sequence of steps where,in a preferred embodiment, the silicon electrode contains a set ofaccess ports illustrated previously in FIG. 13. The resulting sensor(shown in FIG. 16) utilizes the enzyme glucose-oxidase immobilizedcovalently in the gel film, with microparticles functionalized or loadedwith pH-sensitive or oxygen-sensitive fluorescent dyes.

-   -   1—inject solution containing        -   functionalized particles            -   displaying pH-sensitive or oxygen-sensitive dyes known                to the art        -   reaction mixture containing precursors and ingredients for            gel formation        -   functionalized glucose oxidase    -   2—apply AC electric field to trigger LEAPS and produce        microparticle array(s)    -   3—trigger gel formation by UV-initiation of polymerization        -   to form patterned or monolithic gel film incorporating            functionalized glucose oxidase    -   4—remove electric field and UV illumination    -   5—inject (glucose-containing) sample into space below patterned        silicon chip to initiate GOx        diffusion of sample into gel matrix; in the presence of glucose,        the following reaction occurs

D-Glucose+Oxygen D-gluconolactone+Hydrogen Peroxide

-   -   6—monitor reaction shown below by recording fluorescence        intensity from microparticle array; reduced oxygen levels or the        reduced pH in the local gel environment, and their effect on the        bead-anchored dyes, serve as an indirect indication of glucose        concentration.

Example 15 Fabrication of a Gel-Embedded Planar Array of Vesicles

There is a growing interest in developing miniaturized sensing, samplingand signal amplifying structures coupled with an analytical measuringelement to carry out a variety of bioassays. The sensing componenttypically reacts or interacts with an analyte of interest to produce aresponse that can be quantified by an electrical or optical transducer.The most common configurations use immobilized biomolecules on solidphase supports while another less common approach uses livingmicroorganisms or cells or tissues as the sensing structure.

Unilamellar vesicles are composed of a single lipid bilayer shell thatencloses an entrapped aqueous compartment; methods have been describedto prepare giant unilamellar vesicles whose size approaches that ofcells. Such vesicles are attractive as ultra-small reaction vessels or“artificial organelles” in which the reaction is confined and separatedfrom the external medium. Vesicles containing reconstituted integralmembrane proteins provide a synthetic chemical structure to study thefunction of such proteins including many cell surface receptors. Inaddition, the surface of such vesicles can be decorated with a varietyof receptor moieties mimicking a natural cell and allowing complexbiochemical reactions and/or interactions to be studied (Lasic, D. D.Ed. “Liposomes: From Physics to Applications”, 1^(st) ed., ElsevierScience B. V.: Amsterdam, 1993.)

Given a mixture of vesicles of two types, each containing one of thereactants of a reaction of type A+B-->C, two of vesicles of differenttype may be brought into close proximity, fore example, by forming aclose-packed planar array, and may then be fused using a pulsed electricfield in accordance with methods known to the art, in order to form alarger vesicle in which the reaction A+B->C is now performed. In apreferred embodiment, A may represent an enzyme, B a substrate, C theproduct of the enzyme-catalyzed reaction. This reaction scheme may begeneralized to involve more than two reactants.

Vesicles entrapping a single functionalized and encoded microparticlecan be prepared by methods known to the art. Using methods of thepresent invention, microparticle encoded, gel-embedded vesicle arraysmay be prepared (see Examples 1, 2, 3 and 4) to provide for a syntheticassay format in which the function of multiple cell-surface receptorssuch as ion channels may be quantitatively characterized.

A variety of complex biochemical assays may be performed using such acomposite structure. As illustrated in FIG. 17, an array of vesiclesdisplaying multiple types of receptors, each vesicle displaying only onetype of receptor and containing a corresponding fluorescently stainedand functionalized microparticle, is immobilized in a thin gel filmusing methods disclosed herein; the fluorescent color of the particle isused to determine the identity of the receptor on the vesicle. Inaddition, the microparticle, is also functionalized on its surface witha measuring element such as an environmentally sensitive fluorescent dyeto indicate a change in the internal aqueous compartment of the vesiclefollowing a binding event on its surface.

Example 16 Gel-Embedded Cellular Arrays and their Use in Cell-BasedFunctional Assays

The entrapment and immobilization of viable cells in various polymericmatrices, natural or synthetic, including polyacrylamide (Vorlop, K. etal. Biotechnol. Tech. 6:483 (1992)) have been reported, primarily inconnection with biocatalysis (Willaert, P. G. et al. (Eds.),“Immobilized living cell systems: Modeling and experimental methods.”Wiley, New York, 1996). Polymeric matrices can provide a hydratedenvironment containing nutrients and cofactors needed for cellularactivity and growth. To minimize mass transfer limitations, methods ofthe present invention may be used to immobilize arrays of cells in athin and porous gel film.

In accordance with the methods of the present invention, the process offorming a composite structure containing cell arrays entrapped in apatterned or monolithic gel film consists of two stages. First, orderedcell arrays are formed from a cell suspension also containing allingredients required for subsequent in-situ gel formation in accordancewith Example 1. In a preferred embodiment of the array assembly process,LEAPS (see Example 1) is invoked to form arrays from cells suspended ina low viscosity monomer(s) dispersion mixed with an initiator inaccordance with Example 1. Second, gels films are formed, either viaheat-initiated in-situ polymerization to form a spatially patternedcomposite or via UV-initiated in-situ polymerization to form amonolithic composite, as described (see Example 2).

The immobilized cell array system of the instant invention is useful fora variety of assay formats. For example, to analyze and quantify severalmolecular targets within a sample substance, the methods of the presentinvention provides for the means to form a gel-embedded cell arraydisplaying a plurality of receptors (to one or more of the targets)which may be exposed to the sample substance.

An alternative format of a functional assay, shown in FIG. 18, involvesthe combination of a gel-microparticle heterostructure with agel-embedded cellular array prepared by the methods of the presentinvention. Embedding of cells within a thin gel film facilitates theengineering of small, functionally organized heterostructures byavoiding the manipulation of individual cells while providing localchemistries maintaining cells in their requisite environment. Thelateral spacing of cells as well as microparticles within theirrespective arrays is readily tuned in such a structure using LEAPS asdisclosed herein.

Two separate gel films, one containing a functionalized microparticlearray and the other a cellular array, are placed in direct contact in asandwich geometry. In this configuration, particles and cells form pairsof sources and detectors of molecules to be analyzed. For example, cellscan secrete molecules such as cytokines, and proximal beads within thebead array can be designed to monitor the profile, for example in adisplacement assay. Alternatively, small molecules can bephotochemically cleaved from an array of color-encoded beads and can bedetected by monitoring the functional response of cells within theapposed gel-embedded array. The lateral patterning of the arrays as wellas the short diffusion length in the vertical direction helps to preventlateral mixing of the ligand molecules and hence enables execution andsmonitoring of complex local binding chemistries.

Example 17 Characterization and Control of Diffusive Transport in Gels

The diffusion of fluorescently tagged molecules into the gels of thepresent invention were studied using a sandwich cell device asillustrated in FIG. 19 (a). To provide actual chemical anchoring of thegel to both the Si-chip surface and the glass coverslip both of themwere pretreated using vinylmethoxysilaoxane oligomer for polyacrylamidegels, and 3-(glycidoxypropyl)-trimethoxysilane for agarose gel,respectively.

For the coating reaction a 95% ethanol and 5% water solution wasadjusted to pH 5 with acetic acid. The silane coupling agent was thenadded to yield 2 wt % solution. Substrates (chips and cover glasses)were dipped into the solution with gentle agitation for 5 minutes.Following, the substrates were removed from the solution and rinsedbriefly in ethanol. The treated substrates were cured at roomtemperature 24 hours.

For the formation of the acrylamide gels the monomer mixture of 10%(w/v) acrylamide, 3% (w/v) N,N′-methylene-bis-acrylamide (Polysciences,Ltd, USA), 0.1% photo initiator1-[4-2-Hydroxyethoxy)-phenyl]2-hydroxy-2-methyl-1-propane-1-one(IRGACURE® 2959, Ciba Specialty Chemicals (USA)) as well as H₂O wasinjected into the sandwich cell. The masked cell was then exposed to anUV light source (150 W Hg) through a photo-mask for durations from 45 sup to 180 s. Following the exposure, the unpolymerized solution wasremoved from the cell.

For agarose gel formation, 1 μl an agarose solution (0.5% w/v) (heatedto ˜90 C) was carefully pipetted on the surface of a pretreated Si chip,and gently covered with a pretreated cover glass slide. Under theseconditions the drop of the agarose sol deforms approximately into acylindrical plug sandwiched between the two surfaces, which turns into agel under the room temperature conditions within 1-2 minutes. Onceformed, the gel was left undisturbed at room temperature for additional2-3 hours to promote the covalent crosslinking between the hydroxylgroups in the agarose chains and the epoxy group present on thepretreated surfaces.

Although a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will appreciate thatmany modifications of the preferred embodiments are possible using thenovel teachings and advantages of this invention. Accordingly all suchinventions are intended to be included within the scope of thisinvention, as defined in the following claims.

1-81. (canceled)
 82. A method of producing an organized assembly ofparticles by transforming a homogeneous fluid mixture or suspensioncomprising a gellable component and a plurality of particles within areactor, into one or more heterogeneous assemblies, the methodcomprising the steps of: (a) actively forming a spatial arrangement of aplurality of particles in designated regions of one or more boundingsurfaces of a reactor, wherein the active formation is mediated by anexternal field and sustained in said arrangement after the formation bysaid field; and (b) forming a gel, in the presence of the externalfield, to form a gel-microparticle composite.
 83. The method of claim82, further comprising the step of modifying said particles.
 84. Themethod of claim 83, wherein the modification comprises attachment ofbiomolecules to said particle surfaces.
 85. The method of claim 82,further comprising the step of modifying the gel.
 86. The method ofclaim 85, wherein modifying the gel comprises functionalization of thegel by covalent attachment of biomolecules.
 87. The method of claim 82,wherein the particles or the embedding fluid are magneticallypolarizable, the external field comprises a magnet field, said fieldbeing in a direction substantially normal to one of the reactor boundingsurfaces.
 88. The method of claim 82, wherein the arrangement iscomposed of an arrangement of particles within a plane of the boundingsurface or in linear strings oriented substantially normal to thebounding surface.
 89. The method of claim 82, wherein the activeformation step comprises: providing a first electrode positioned in afirst plane, and a second electrode positioned in a second planedifferent from the first plane, providing a polymerization mixturecomprising a monomer and an initiator in an electrolyte solution whereinsaid polymerization mixture is located between the first and the secondelectrode; providing a plurality of particles suspended in saidsolution; generating an AC electric field with a field distribution atan interface between said first electrode and said electrolyte solution;and polymerizing the polymerization mixture to form a polymeric film,wherein the polymerization step results in formation of apolymer-particle composite, said composite comprising the assembly ofparticles embedded in the polymeric film.
 90. A polymer particlecomposite comprising an array of particles at a surface of said polymer,wherein said surface is formed by cleaving and removing a layer ofpolymer thereby exposing said particle containing surface.
 91. A polymerparticle composite comprising an array of particles at a surface of saidpolymer wherein said surface is formed by detaching the polymer particlecomposite from a substrate to expose said particle containing surface.